Fatigue, due to the sustained mental effort required to process and understand speech, is a common complaint of people with hearing loss and one that can have a significant negative effect on quality of life. Mental fatigue decreases our abilit to maintain attention and concentration, slows mental processing, and impairs decision-making. Fatigued adults in the workplace are less productive and more prone to accidents, and adults with fatigue are less active, more socially isolated, less able to monitor their own self care, and more prone to depression than non-fatigued adults. Although reports of fatigue among persons with hearing loss are common, the relationship between hearing loss and fatigue is largely unexplored. A primary barrier to investigating the linkage between hearing loss and mental fatigue is the lack of an objective measure of fatigue associated with speech processing. To address these limitations the specific aims of this research are to 1) identify optimal test conditions to objectively quantify mental fatigue resulting from speech understanding difficulties and 2) define relationships between degree of hearing loss and mental fatigue. To achieve the first specific aim we will monitor mental fatigue, using objective and subjective metrics, while participants complete a speech monitoring task we have created. We will systematically vary signal-to- noise ratio (SNR) and test duration to find the optimal SNR|task duration combination for inducing mental fatigue due to sustained speech processing demands in listeners with hearing loss. To achieve our second specific aim we will use subjective and objective metrics to compare mental fatigue resulting from our speech monitoring task (using the optimal SNR|task duration parameters), across three groups of adults that vary in terms of degree of hearing loss; a) normal hearing for their age, b) mild-moderate hearing impairment, and c) severe hearing impairment. The outcomes of this research have important short, and long, term implications. An immediate outcome will be an optimized, objective, procedure for quantifying mental fatigue resulting from speech processing demands. Currently no such procedure exists and this lack is a significant barrier to advancing understanding in this important, but previously ignored, area. Additionally, this research will define the relationship between degree of hearing loss and mental fatigue, a critical first step towards identification of those at most risk of the negative consequences of fatigue on quality of life. In the long term, we expect this objective procedure to be useful both as a research tool and for practical clinical applications such as hearing aid counseling and evaluation of interventions designed to reduce fatigue in this population.